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June 22, 2010 -- Progesterone alone, when given in a form that's identical to the natural hormone, helps relieve the hot flashes and night sweats experienced by women during menopause, according to a new study.

The form of progesterone studied is known as oral micronized progesterone, says Jerilynn Prior, MD, professor of endocrinology and metabolism at the University of British Columbia, Vancouver, who presented her findings Monday at ENDO 2010, the annual meeting of The Endocrine Society, in San Diego.

''It's molecularly identical to what our own bodies make," she says of the progesterone studied. When she compared this progesterone for hot flashes to placebo, it came out on top. The regimen may prove to be a practical alternative to traditional hormone therapy, which combines progesterone and estrogen, the research suggests, especially for women who do not want to take estrogen or should not because of their medical history.

"Oral micronized progesterone is a highly effective treatment for hot flashes and night sweats," Prior says. The hormone is sold only by prescription and goes by the brand name Prometrium in the U.S. and Canada. It is synthesized from yams, and the micronization is meant to make it easier to absorb.

Progesterone for Hot Flashes: A Closer Look

Prior and her colleagues assigned 114 postmenopausal women who had hot flashes and night sweats to one of two groups. One group took 300 milligrams of Prometrium daily. The other took a placebo. Neither group knew whether they were taking Prometrium or the placebo.

The time since the last menstrual flow varied from one to 10 years and on average was four years. If the women had taken hormone therapy in the past, they had to have discontinued it for the past six months to be eligible to enter the study.

The women noted the number and intensity of symptoms in a daily diary, and then Prior calculated the average daily score that reflected the number and intensity of symptoms.

At the end of the 12-week study, Prior says, "Prometrium was significantly more effective than the placebo.''

The women taking Prometrium had a 56% decrease in their score, and the women taking the placebo had a 28% decrease.

The women taking Prometrium had a 48% decline in the number of symptoms; women taking the placebo had a 22% decline.

None of the women reported any serious adverse events, Prior says.

Exactly how progesterone relieves the symptoms isn't clear, Prior tells WebMD. "We know progesterone does lots of calming things in the brain. It may compensate for something estrogen does [to reduce the hot flashes]."

'Natural' Progesterone for Hot Flashes

The results are ''moderately impressive," says Carolyn Alexander, MD, a reproductive endocrinologist and infertility specialist at Cedars-Sinai Medical Center and assistant professor of obstetrics and gynecology at the University of California Los Angeles David Geffen School of Medicine, who reviewed the study results for WebMD.

''Using Prometrium alone may be considered an interesting new use of the medication," she tells WebMD.

Though it has been approved by the FDA since 1998, the new study is believed to be the first to look at micronized progesterone alone for hot flash and night sweat relief, Prior says.

Alexander cautions that more study is needed. What is yet to be proven, she says, is whether the regimen will be safe long-term for bone health. She says researchers also need to show no increased risk for cancers.

Addressing Bone, Cancer Concerns

Prior agrees more study is needed but suspects the micronized progesterone will not adversely affect bone health, pointing to previous research suggesting that estrogen and progesterone when given together improve bone health more than estrogen alone.

The micronized progesterone alone regimen may actually decrease cancer risk, she says, although that's not proven. She points to research showing that estrogen is involved in cell growth (including cancer cells) and progesterone is involved in cell maturation and differentiation. "That would be expected to decrease cancer risk," she says.

On the Prometrium web site, the manufacturer, Abbott, cautions women taking the hormone for the indicated uses to have a yearly exam by their health care provider ''due to increased risk of breast cancer while using this medication."

The study was funded by donations to the Centre for Menstrual Cycle and Ovulation Research at the university; Schering Canada and Besins Healthcare provided the progesterone and placebo.

This study was presented at a medical conference. The findings should be considered preliminary as they have not yet undergone the "peer review" process, in which outside experts scrutinize the data prior to publication in a medical journal.